Extreme Reach Inc., a Needham-based video platform for integrated TV, online, and mobile advertising, said that it has agreed to buy the TV business of Digital Generation Inc., including its advertising distribution business unit, for $485 million in cash.

“Our vision from day one has been to power digital video advertising,” Extreme Reach chief executive John Roland said in a statement. “This acquisition is a major step in that direction.”

The company tells its customers that with Extreme Reach, they will be able to manage, deliver, and measure ad campaigns “across every screen.”

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The acquisition is expected to be financed by a combination of existing cash, debt financing to be arranged by J.P. Morgan and SunTrust Robinson Humphrey, and new equity from existing investors of Extreme Reach, the company’s press release said. Spectrum Equity, which invested $51 million in Extreme Reach in May 2013, has agreed to invest up to an additional $47 million to facilitate the acquisition.

In addition to DGIT shareholder approval, the transaction is subject to various other terms and conditions, including approval under the Hart-Scott-Rodino Antitrust Improvements Act. The transaction is expected to close in the first quarter of 2014.

In May, when Spectrum Equity’s investment was announced, Extreme Reach said it had 225 employees and was on pace to exceed an annual revenue run rate of $100 million by year’s end.